Offshore wind energy is becoming more and more important. In particular, in the last couple of years, there has been an enormous increase in the number of wind farms (a grid of windmills placed on the continental shelf in the seas). Not only is the number of offshore windmills increasing, but also their size, i.e. the windmills are getting bigger. The latter brings new challenges in terms of actually installing these offshore windmills. Dedicated huge ships have been built having very large heavy duty cranes placed on top of them. The general idea is to manufacture the windmills onshore in as large parts as possible and then transport these parts to the desired location, where they are placed using the ever-increasing heavy-duty cranes on an earlier prepared offshore windmill pedestal. There exists a huge variety of different technologies of preparing and building such windmill pedestals offshore, but that is beyond the scope of what is claimed herein as an invention. As the size of the offshore windmills increases further, also the size of these dedicated windmill installation ships is increasing. In order to facilitate the use of a huge crane these ships are provided with a jack-up system such that they lift themselves out of the sea, which makes the ships orientation independent of the waves. It is particularly the operational costs of these ships, which is of concern.
In the prior art some suggestions are reported to resolve this issue.
GB2,365,905A discloses an offshore structure comprising a watertight chamber having a base, which rests on the seabed and a telescopically extendable shaft, which extends upwardly from the chamber, with a hoist located at the shaft top. A wind turbine may be located at the top of the shaft, while a generator may be located in the base. The structure may be towed floating with the column retracted to its installation point, where the base is then ballasted to its resting position on the seabed. The column may then be extended and grouted in place, while the hoist may be used to raise the rotor hub and blades to the required position at the top of the column.
WO2010/151145A1 discloses a windmill comprising a generator house with a generator (also being referred to as “nacelle”) and a plurality of rotor blades at an upper part of an upright shaft. Said shaft comprises at least one telescopic joint for altering the height of said windmill, and further comprises a pivotal connection of the blades between a substantially vertical and a substantially horizontal position. The respective disclosure relates to a method of installation, intervention or decommissioning of said windmill. It finds application to fixed, bottom mounted offshore windmills or onshore windmills.
A first disadvantage of the above-presented windmills and installation methods is that they are not retrofit solutions, i.e. they cannot be applied to existing offshore windmill pedestals. In addition, said methods and windmills are not easily scaled up. Thus, there is a need for a further improved retrofit method of building offshore windmills.